The present invention generally relates to a switched capacitor filter (which will be sometimes referred to merely as the SCF, hereinafter) which is used in a modulator or the like for a digital mobile radio telephone system or a digital portable telephone. The present invention also relates to a circuit which employs the SCF.
FIG. 1 schematically shows an arrangement of a prior art SCF circuit. The SCF circuit of the drawing includes an analog signal input terminal 201, a clock input terminal 202, a clock generator 203, an SCF 204, a sample holding circuit 205, a low pass type analog filter 206 and an analog signal output terminal 207.
The operation of the above prior art SCF circuit will be explained. The SCF circuit operates in such a manner that its output spectrum has such a route Nyquist characteristic (.alpha.=0.5) as shown in the following equation (1) with respect to a 21 kHz rectangular pulse train input. ##EQU1##
In FIG. 1, a system clock having a frequency of 5.376 MHz is applied to the clock input terminal 202 and the clock generator 203 generates clock signals necessary for the operation of the SCF 204 and sample holding circuit 205 on the basis of the system clock received from the input terminal 202.
When a 21 kHz rectangular pulse train is applied to the analog signal input terminal 201, the SCF 204 samples the amplitude value of the received analog input signal at 336 kHz, fetches therein and then performs its filtering operation. Further, an output of the SCF 204 is held at the sample holding circuit 205 with respect to 0-the order to have such an output waveform as shown in FIG. 2.
The 0-th-held output spectrum, which expressed by the following equation (2), contains unnecessary frequency components which have frequencies corresponding to a multiple of the sampling frequency 336 kHz as shown in FIG. 3. ##EQU2## where fs=336 kHz.
The analog filter 206, which has a third-order Butterworth characteristic with a cut-off frequency of 50 kHz, suppresses the aforementioned unnecessary spectrum components and then outputs it to the analog output terminal 207. An example of the analog filter 206 is shown in FIG. 4.
However, the above prior art SCF circuit has had such a problem that such an analog filter having a low cut-off frequency and an abrupt characteristic as shown in FIG. 4 is indispensable as a post-filter. This makes it difficult to make the SCF circuit in the form of an integrated circuit (IC) to eliminate the need for any adjustment.
Further, when the circuit is simplified by reducing the pass frequency order of the post-filter, the sampling frequency must be set to be high, which undesirably increases the power consumption of the SCF of the main side (with respect to the post-filter).